1. Field of the Invention
This invention relates generally to an injection molding method and apparatus, specifically to a method and apparatus for generating an injection speed pattern.
2. Description of the Related Art
In injection molding machines, the injection speed pattern, through which the speed an injection screw is controlled during an injection operation, depends greatly on the shape of a molded article.
In the conventional method, however, the following procedures are necessary to determine the injection speed pattern:    (A) defining a filling stroke of the injection screw in a barrel cylinder, determined by dividing the volumetric capacity of the molded article with the diameter of the barrel,    (B) defining an injection speed in relation to the filling stroke through empirical knowledge, and then actually performing injection molding using the defined injection speed,    (C) modifying various molding conditions in accordance with the article molded in (B), and    (D) obtaining optimal molding conditions.
Accordingly, long-term experience and skill in the field of injection molding are required to determine the injection speed pattern of a given molded article.
FIG. 7 illustrates a cross sectional view of a pair of mold dies for molding a solid article with relatively simple shape.
As shown in FIG. 7, the mold dies MLD define a cavity CV which is filled with molten resin which then solidifies , thereby forming a molded article W.
The molten resin is supplied from a barrel BRL into the cavity CV through a nozzle NZ and a gate GT by means of an injection screw SCR.
The stroke of the injection screw SCR in the direction X of the injection screw axis is determined as indicated above, such that the volume of the resin in the barrel BRL after retracting the injection screw is equal to that of the molded article W, namely the capacity of the cavity CV.
Then, the molded article W is divided in the direction X into zones Z0, Z1, Z2, Z3, Z4 and Z5, where zone Z0 corresponds to gate GT. During injection, resin RS reaches zone Z1 from zone Z0, and in zone Z1, the moving speed of the injection screw SCR is so controlled to gradually increase the rate at which resin RS is supplied. A similar increasing of the rate at which resin is supplied is performed from zone Z2 to zone Z3. After zone Z3, the moving speed of the injection screw SCR is so controlled to gradually decrease the rate at which resin RS is supplied.
In contrast, in case of a cylindrical molded article Wa, shown in chained line in FIG. 7, the moving speed of the injection screw SCR is held constant from zone Z1 through zone Z5.
Thus, in general, the moving speed of the injection screw in a large cross-sectional area zone must be faster than that in a small cross-sectional area zone. This is the general understanding of the filling process.
However, even with such understanding, it is very difficult, particularly for an unskilled person, to determine both the series of injection speeds of the screw specifically as numerical values and the positions at which the injection speed should change. Consequently, many trials are necessary to improve the injection speed pattern.
Therefore, automatic injection speed pattern generation technology has been desired.
Automatic injection speed pattern generation technology is disclosed, for example, in Japanese laid open (unexamined) patent number 9-267374, which teaches that the shape of a molded article is resolved into micro elements and a numerical analysis method is applied to the micro elements. However, the numerical analysis method is disadvantageous in that it requires complex calculation.